In this FIRCA we propose an in-depth analysis of chromatin organization at Mu transposons and cellular genes after UV-B irradiation. The focus is on establishing whether UV-B alters the abundance or post- translational modification of chromatin remodeling proteins and other DNA-associated proteins. Next we will examine the chromatin proteins bound at Mu elements and several host genes that are up- or down- regulated by UV-B treatments. In this study we will ask whether there are changes at particular genes and whether similar changes occur at genes whose transcript increase (or decrease) during treatment. Of paramount importance to the project is examining changes that occur at silenced Mu elements that might explain how UV-B can reactivate these transposons. Well-defined UV-B treatments and appropriate genotypes with well-documented UV-B responses are employed in each experiment permitting a robust test of each hypothesis. The methods used will include proteomics to define nuclear protein abundances and post-translational modifications following UV-B exposure, ChIP to define chromatin factors differentially associated with active and silent Mu elements before and after UV-B exposure, and selective microarray hybridization and RT-PCR assays to monitor transcript changes in RNAi knockdown stocks that are extremely UV-B sensitive. The studies are designed to address the fundamental question of how a variable environmental factor perturbs cellular gene expression, not only at the site of radiation but in distant tissues. Given the extensive parallels between cellular responses in animals and plants, these data should prove useful in understanding the responses of human cells to the challenge of environmental UV-B exposure. As ozone thinning in the Southern hemisphere impacts Argentina directly, there is heightened interest in understanding UV-B damage to people, major crops such as corn, and the entire ecosystem. Even low UV-B exposure harms organisms: it is a powerful carcinogen, implicated in skin cancer in humans. Land plants are particularly sensitive, because photosynthetic tissues are inevitably exposed to UV-B. Thus studies of plant damage can help define repair pathways required to maintain cellular functions in other eukaryotes. [unreadable] [unreadable]